VR performance with Cesium for Unity

There seems to be a big performance hit when loading a Cesium for Unity scene in VR, at least if it's using the XR Interaction toolkit.

This can be observed in the VR01 sample scene - opening the scene immediately spikes CPU usage from a baseline Cesium for Unity scene (<1%) to over 90% on my machine. Deleting the Cesium georeference and 3D tilesets from the scene immediately reduces CPU usage.

Additional profiling is needed to help narrow down the cause of the issue.

Start with the VR01 sample scene and reproduce the CPU spike using the XR Interaction toolkit, comparing it with a baseline Cesium for Unity scene. Profile the scene while the Cesium georeference and 3D tilesets are present, then narrow down the cause; done means the source of the performance hit is identified and a verified improvement is documented.

Written by the indexing model from the issue text.